1. Field of the Invention
This invention relates generally to flip-flops and, more particularly, to a TTL flip-flop having improved AC and DC characteristics including a quicker output transition from a high state to a low state and a greater resistance to output degradation.
2. Background Art
Master-slave flip-flops typically include a data gate, a master section, and a slave section. The data gate receives an input signal and provides internal signals to the master section. The master section is responsive to a clock signal and generates first and second signals. The slave section is responsive to the first and second signals for storage therein and generates traditional Q and Q outputs.
The slave section of a previously known flip-flop is shown in FIG. 1 and is also described in detail in the Detailed Description of the Invention. This previously known flip-flop comprises a latch means coupled between the master section and output buffers that provide the Q and Q outputs. The latch means includes a pair of differentially connected transistors having their collectors cross-coupled to a respective control means. Each control means comprises a first transistor having a collector connected to a base of one of the differentially connected transistors and a first emitter connected to the first signal from the master section and to a first emitter of a second transistor. The second emitters of the first and second transistors are both connected to the collector of the other differentially connected transistor. The collector of the second transistor of one control means is connected to an output buffer for providing an output.
However, this previously known circuit has certain DC and AC disadvantages. When the second transistor of the control means is off, it is possible for a leakage current through its collector to drive the output buffer causing the output to degrade, or fail to hold a constant high level. The possibility of this leakage increases as the temperature increases and is more critical at military specifications.
Furthermore, when the output switches from a high state to a low state, the diffusion capacitance of the base-emitter of the second transistor must be discharged, the base-collector capacitance of a second transistor must be charged, and the base capacitance of a transistor in the output buffer must be charged.
Therefore, what is needed is a flip-flop having an improved DC characteristic wherein the sensitivity to output degradation is reduced and an improved AC characteristic wherein the high state to low state output transition time is improved.